When medicinal doses are delivered to patients by infusion, it is sometimes desirable to deliver the medicinal dose at an initially high rate and then deliver the remaining dose at one or more stepped-down lower rates. For example, it is typically desirable for an initial flow for drug infusion to be substantially higher than the desired therapeutic rate, so as to rapidly increase the blood concentration into the desired therapeutic range. This initial high rate of flow is called the “bolus rate”. Once the drug concentration has been increased into the therapeutic range, the flow rate is dropped to the rate necessary to maintain the concentration of the drug in the therapeutic range. This latter flow rate is called the “basal rate”.
Prior to the present invention described below, to achieve a stepped adjustment of the flow rate automatically, an infusion device with an electronically-controlled pump was required. Accordingly, there is a need for a non-electronic infusion device of a simple mechanical construction which does not require a pump, and which can automatically deliver drugs to a patient by way of infusion at an initial high infusion rate, followed by one or more stepped-down lower infusion rates.